1. Field of the Invention
The invention concerns a device for detecting the angular position of a rotor of a polyphase rotary electrical machine containing a stator.
The invention also concerns a rotary electrical machine containing such a device.
It is especially suitable for reversible machines, known as alternator-starters, which are used in the vehicle industry, both in alternator mode and in starter mode, or as an aid to moving off (boost mode), typically from 500 rpm.
2. Description of Related Art
Within the context of the invention, the term “polyphase” concerns more specifically three-phase or hexaphase rotary electrical machines, but may also concern biphase rotary electrical machines or those which operate at a higher number of phases.
For the sake of clarity, the following scenario relates to the preferred application of the invention, i.e. the case of a reversible three-phase rotary electrical machine of the alternator-starter type, without this in any way limiting the scope of the invention.
As is well known, a reversible rotary electrical machine contains an alternator comprising:                a rotor constituting an inductor, traditionally combined with two collector rings and two brushes to supply an excitation current; and        a polyphase stator, bearing several coils or windings, three in the embodiment in question, constituting an armature, which are star-connected, or most often as a triangle in the case of a three-phase structure, and which deliver converted electrical power to a bridge rectifier when operating as an alternator. The machine includes two bearings, a front and a rear, to fix it to the thermal engine and to fix the stator. The stator surrounds the rotor. The rotor is carried by a shaft supported by the front and rear bearings. The brushes of an excitation circuit of the rotor are powered by a regulator of the alternator to maintain the output voltage of the alternator at a desired voltage to supply an electrical supply network containing a battery. The alternator enables any rotation movement of the inductor rotor driven by the thermal engine of the vehicle to be transformed into an electrical current induced in the coils of the stator.        
The alternator may also be reversible and constitute an electric motor, or rotary electrical machine, enabling the thermal engine of the vehicle to be driven in rotation via the rotor shaft. This reversible alternator is known as an alternator-starter, or alterno-starter. It enables mechanical energy to be transformed into electrical energy, and vice versa.
Thus, in alternator mode, the alternator-starter specifically charges the vehicle battery, while in starter mode, the alternator-starter turns the motor vehicle's thermal engine, also known as internal combustion engine, in order to start it.
In reversible machines from the automotive industry, for example, operating in motor or starter modes, the stator must be current-controlled in such a way that at any moment the necessary torque can be applied to the rotor to impel the required rotation for the operation of the engine. The torque applied to the rotor, and hence the current supplied to the phases of the stator, is a sinusoidal function of the angular position of the rotor in relation to the stator, represented by an angle θ.
FIG. 1, placed at the end of this description, illustrates in diagram form a complete system 1 for detecting the angular position θ(t) of the rotor of a three-phase alternator-starter and for controlling said organ, either in alternator mode or in engine (starter) mode.
The system 1 consists of four principal sub-systems: an alternator-starter 10, a reversible AC-DC power converter 11, a control module 13 for this converter and a module 12 for detecting the angular position ƒ of the rotor 100 (symbolised by an arrow turning about its axis of rotation Δ).
The converter 11 generally consists of a bridge of electronic rectifiers, comprising three banks of MOSFET power transistors, under the single reference 110, one for each phase. A structure of this type is well known to the person skilled in the art and there is no need to describe it in further detail.
In alternator mode, the alternator-starter 10 supplies the converter 11 with three-phase AC current via its three outputs, 101 to 103, which correspond to the junctions between the three coils (not shown in FIG. 1) constituting the stator 104 of the alternator-starter 10. The latter converts the three-phase AC current into DC current so as to (re)charge the battery Bat with which the vehicle is equipped (not shown in FIG. 1). This, in turn, supplies various organs of this vehicle: on-board electronics, air conditioning, headlights, etc.
In engine mode, i.e. in starter mode, the alternator-starter 10 is supplied with three-phase electrical energy by the reversible converter 11, which is operating in three-phase current generator mode.
Whichever mode is considered, the MOSFET transistors 110 are controlled according to an appropriate sequence of six control signals, SC1 to SC6, generated by the control module 13. As is also well known, these signals SC1 to SC6 must be generated synchronously with the angular position θ of the rotor 100 which detects the relative phases of the currents supplied by the outputs 101 to 103 of the alternator-starter 10.
For this reason, it is necessary to detect this angular position θ with great precision, in order to achieve correct functioning of the bridge rectifiers, in particular to avoid any risk of deterioration of the semiconductor components, but also and above all, in engine or starter mode, to optimise the torque supplied by the alternator-starter 10.
This is the function which is devolved to the module 12 for detecting the angular position θ of the rotor 100 so as to generate a signal θ(t) representing the instantaneous variation of the measured angular position and to transmit it as an input to the control module 13.
In prior art, various methods have been proposed for this purpose.
Certain reversible electrical machines, especially those used in the automotive industry, are now equipped with a device known as a resolver, which is positioned at the end of the rotor shaft of the machine. Such a resolver is described, by way of non-exhaustive example, in the patent application US 2002/0063491 A1. The resolver itself contains a stator and a rotor which are fixed in relation to the respective stator and rotor of the reversible machine. The resolver measures the magnetic field produced by its own rotor. As this magnetic field is fixed in relation to said rotor, which is itself fixed in relation to the rotor of the machine, it represents the position of the actual rotor of the machine.
However, this type of equipment presents a certain number of disadvantages, in particular the following:                resolvers are quite expensive, and, to render them operational, their implementation is complex, due to the coupling to be effected between the resolver and the actual reversible machine, which necessitates the presence of an electronic calculation component to provide the correct position of the rotor of the reversible machine based on the coupling parameters.        Secondly, resolvers are sensitive to the magnetic interference caused by the stray magnetic field produced by the rotor, which causes malfunctioning of the system, thus errors of measurement and poor control of the machine. To limit this disadvantage, it is necessary to use magnetic protection, such as a stainless steel tube placed between the rotor and the resolver at the shaft end. Moreover, the mechanical strength of these devices is imperfect, since they are especially sensitive to vibrations from the machine due to being mounted on the shaft end of the rotor. Moreover, their size is a problem and is unlikely to enable greater compactness of the electrical machine.        Finally, their resistance to salt spray and dust is not completely satisfactory.        
To alleviate these disadvantages, in international patent application WO 2006/010864 A2, the claimant proposed a device for detecting the position of a rotor of a rotary electrical machine containing a stator, which makes it possible to obtain the precise angular position sought, while at the same time being cheap, simple to operate and having low sensitivity to magnetic interference.